How age changes affect the cellular kinetic behavior during growth, development, maintenance and repair potentials of skeletal tissues is a basic question of paramount importance. To this end these studies are designed to determine age changes in the cellular and matrical complement of skeletal tissues in both normal and injured mouse femora. A variety of techniques encompassing morphological, autoradiographic and histo-cytochemical methods at light-microscopic, as well as, electron- microscopic levels are employed. In order to develop a better understanding of changes in the cellular proliferative kinetic, cellular behavior and repair potentials of aging skeletal cell systems, tritiated compounds, such as tritiated thymidine, tritiated uridine, tritiated amino acids and tritiated carbohydrates are used. The data collected and knowledge gained from these studies will serve as a base line for a wide variety of future undertakings devoted to assessing the response and behavior of bone to perturbations such as radiation injury, nutrition and hormonal variations, and physical, chemical and bacterial trauma, concomitant with aging.